Bortezomib Enhances the Efficacy of Fulvestrant by Amplifying the Aggregation of the Estrogen Receptor, Which Leads to a Proapoptotic Unfolded Protein Response

Abstract

Purpose Fulvestrant is known to promote the degradation of the estrogen receptor (ER) in the nucleus. However, fulvestrant also promotes the aggregation of the newly synthesized ER in the cytoplasm. Accumulation of protein aggregates leads to cell death but this effect is limited as a result of their elimination by the proteasome. We tested whether combining fulvestrant with the proteasome inhibitor, bortezomib, could enhance the accumulation of ER aggregates and cause apoptotic cell death. Experimental Design The rate of aggregation of the ER was monitored in ER+ breast cancer cells lines, T47D, ZR-75.1, BT474, MDA-MB-361, MCF-7, fulvestrant resistance MCF-7, and tamoxifen-resistant T47D-cyclin D1 cells. Activation of the unfolded protein response, apoptosis, and metabolic rate were also monitored in these cell lines following treatment with fulvestrant, bortezomib, or bortezomib in combination with fulvestrant. Results We found that bortezomib enhances the fulvestrant-mediated aggregation of the ER in the cytoplasm without blocking the degradation of the ER in the nucleus. Further, these aggregates activate a sustained unfolded protein response leading to apoptotic cell death. Further, we show that the combination induced tumor regression in a breast cancer mouse model of tamoxifen resistance. Conclusions Adding bortezomib to fulvestrant enhances its efficacy by taking advantage of the unique ability of fulvestrant to promote cytoplasmic aggregates of the ER. As this effect of fulvestrant is independent of the transcriptional activity of the ER, these results suggest that this novel combination may be effective in breast cancers that are ER+ but estrogen independent.

Estrogen-Dependent Gene Transcription in Human Breast Cancer Cells Relies upon Proteasome-Dependent Monoubiquitination of Histone H2B

“…A recent study reported a combinatorial effect of fulvestrant and bortezomib on the induction of apoptosis and laid the ground work for ongoing clinical trials combining these 2 treatments (16). In this study, we were unable to see any effects of bortezomib treatment on the rapid fulvestrantinduced ERa downregulation and the repression of ERa target gene transcription or ERa binding.…”

“…S1C). Surprisingly, in contrast to previous reports (16,26), whereas bortezomib effectively decreased estrogen-induced proteasomal degradation of ERa ( Supplementary Fig. S1D) in a manner similar to that reported for MG132 (9, 27), we observed little or no effect of bortezomib on fulvestrant-induced ERa downregulation ( Supplementary Fig.…”

“…Although clinical trials for the treatment of metastatic breast cancer were initially disappointing when used as a single agent (14), a new study combining the pure antiestrogen fulvestrant with bortezomib suggests that the combination of antiestrogen therapies with proteasome inhibition may indeed increase treatment efficacy (16). In line with these results, clinical trials utilizing the combination of bortezomib and fulvestrant are currently underway for the treatment of ER-positive metastatic tumors in postmenopausal patients (NCT01142401).…”

Linking autophagy with inflammation through IRF1 signaling in ER+ breast cancer

“…9 Moreover, cytosolic accumulation of ER was previously shown to induce the endoplasmic reticulum stress pathway, the unfolded protein response (UPR) that might link IRF1/ERa/UPR signaling. 10 A pathway schematic illustrating the IRF1/ERa/ATG7 signaling axis is shown in Figure 1. Interestingly, neither the chemical inhibitors HCQ and 3-methyladenine (3-MA) nor treatment with ATG5 siRNA resulted in elevated IRF1 levels in the ERC breast cancer cell lines, suggesting that this relationship is directly mediated by ATG7 or BECN1 protein signaling and not by autophagic flux.…”

Gender Specific Issues in Hereditary Ocular Disorders

“…The clinical data to show that these conditions have been consistently exacerbated or ameliorated under conditions of altered sex hormonal levels is also lacking. [26][27][28][29] Lyonization The most obvious gender-specific differences among monogenic disorders are the differences in prevalence, onset, and severity of clinical manifestations of X-linked inherited conditions. One mechanism that contributes to this variability is X-Chromosomal inactivation (lyonization).…”